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Precautions against West Nile Virus advised for horse owners

May 27, 2005     E-mail this page to a friend!

Department of Livestock, Helena, MT -- Even though mosquitoes and summer picnics may be a few months away, horse owners are encouraged to begin taking precautions against West Nile virus (WNV) now.

“There are USDA licensed vaccines available to help prevent equine cases of WNV encephalitis, and for horses not previously vaccinated, two initial doses given three to six weeks apart are recommended,” said Dr. Tom Linfield, Montana State Veterinarian with the Montana Department of Livestock.

Following the two initial doses of vaccine, the vaccine manufacturers also recommend an annual booster, and some veterinarians recommend two boosters each year, particularly in high-risk areas.

Combination vaccines are also available. Combination products provide protection against multiple diseases including West Nile virus; Eastern, Western or Venezuelan equine encephalitides; and tetanus.

For horses that contract WNV, an equine-origin antibody product that aids in the treatment of horses is also available. Supportive veterinary treatment such as anti-inflammatories, administering fluids and the use of a sling to keep the animal upright have also been used in equines diagnosed with WNV.

Horses appear to be affected by WNV much more often than any other domestic animals. Infection can cause encephalitis, affecting the nervous system, and can cause severe complications and death in horses. The disease also affects humans.

Many horses infected with WNV do not develop any illness, but of horses that become ill, about one-third will die or need to be euthanized.

Although WNV has been documented in the United States since 1999, the first case of WNV in Montana was confirmed in a horse in Yellowstone County in 2002. During that season, 134 equine cases were reported in 26 Montana counties; and 38 of the horses died or were euthanized.

In 2003, WNV was confirmed in 193 horses in 34 Montana counties. A total of 70 horses died or were euthanized. Only 17 of the 193 confirmed horse cases were fully vaccinated.In 2004, WNV was confirmed in 11 horses in seven Montana counties, and five horses died or were euthanized. Only one of the 11 horses was fully vaccinated, and that horse survived.

Montana also confirmed two human cases of WNV in 2002, 228 cases in 2003, and six cases in 2004. The disease is spread through the bite of an infected mosquito. Mosquitoes become infected by feeding on infected birds. The virus does not appear to spread from horse to horse or from horse to person. Person to person spread, though extremely rare, may occur by means of organ transplants or from infected mothers to their infants through breastfeeding.

To diagnose WNV in equines requires a serology test that can be conducted at the Veterinary Diagnostic Laboratory in Bozeman.
Clinical signs of encephalitis in horses include loss of appetite and depression, in addition to any combination of weakness or paralysis of hind limbs, muzzle twitching, impaired vision, incoordination, head pressing, aimless wandering, convulsions, inability to swallow, circling, hyperexcitability, or coma.

“These are also clinical signs of Western and Eastern Equine Encephalitis, viral diseases that affect the nervous system and can cause severe complications and death in horses,” Dr. Linfield said. In addition, rabies cases may also present with similar neurological signs, and should always be considered as a differential diagnosis. These diseases also affect humans.

Vaccination and mosquito control continue to be recommended as methods to help protect horses against the virus. For mosquito control, thoroughly clean livestock watering troughs on a regular basis, remove any potential sources of water in which mosquitoes can breed, dispose of water-holding containers such as discarded tires, and do not allow water to stagnate.

If possible, horses should be stabled inside from dusk to dawn to reduce contact with mosquitoes.

Horse owners are encouraged to contact their local veterinarian for more information regarding WNV vaccine and treatment. More information is also available at the Montana Department of Livestock website at